Recent work in the field of olefin upgrading has resulted in a catalytic process for converting lower olefins to heavier hydrocarbons. Heavy distillate and lubricant range hydrocarbons can be synthesized over ZSM-5 type catalysts at elevated temperature and pressure to provide a product having substantially linear molecular conformations due to the ellipsoidal shape selectivity of certain medium pore catalysts.
Conversion of olefins to gasoline and/or distillate products is disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502 (Givens, Plank and Rosinski) wherein gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed made up of a ZSM-5 type zeolite. Particular interest is shown in a technique developed by Garwood, et al., as disclosed in European Patent Application No. 83301391.5, published Sept. 29, 1983. In U.S. Pat. Nos. 4,150,062; 4,211,640; 4,227,992; and 4,547,613 Garwood, et al. disclose the operating conditions for the Mobil Olefin to Gasoline/Distillate (MOGD) process for selective conversion of C.sub.3.sup.+ olefins to mainly aliphatic hydrocarbons.
In the process for catalytic conversion of olefins to heavier hydrocarbons by catalytic oligomerization using a medium pore shape selective acid crystalline zeolite, process conditions can be varied to favor the formation of hydrocarbons of varying molecular weight. At moderate temperature and relatively high pressure, the conversion conditions favor C.sub.10.sup.+ aliphatic product. Lower olefinic feedstocks containing C.sub.2 -C.sub.8 alkenes may be converted; however, the distillate mode conditions do not convert a major fraction of ethylene. A typical reactive feedstock consists essentially of C.sub.3 -C.sub.6 mono-olefins, with varying amounts of non-reactive paraffins and the like being acceptable components.
Although it is known to use basic materials to deactivate the Bronsted acid sites on the surface of aluminosilicate catalysts (see U.S. Pat. No. 4,520,221 and U.S. Pat. No. 4,568,786, Chen, et al., incorporated herein by reference), the basic materials employed are bulky amines, such as di-tert-butyl pyridine.
Shape-selective oligomerization, as it applies to the conversion of C.sub.2 -C.sub.10 olefins over ZSM-5, is known to produce higher olefins up to C.sub.30 and higher. As reported by Garwood in "Intrazeolite Chemistry 23", (Amer. Chem. Soc., 1983), reaction conditions favoring higher molecular weight product are low temperature (200.degree.-260.degree. C.), elevated pressure (about 2000 kPa or greater), and long contact time (less than 1 WHSV). The reaction under these conditions proceeds through the acid-catalyzed steps of (1) oligomerization, (2) isomerization-cracking to a mixture of intermediate carbon number olefins, and (3) interpolymerization to give a continuous boiling product containing all carbon numbers. The channel systems of medium pore catalysts impose shape-selective constraints on the configuration of the large molecules, accounting for the differences with other catalysts.
The desired oligomerization-polymerization products include C.sub.10.sup.+ substantially linear aliphatic hydrocarbons. This catalytic path for propylene feed provides a long chain which may have one or more lower alkyl (e.g., methyl) substituents along the straight chain.
The final molecular configuration is influenced by the pore structure of the catalyst. For the higher carbon numbers, the structure is primarily a methyl-branched straight olefinic chain, with the maximum cross-section of the chain limited by the dimension of the largest zeolite pore. Although emphasis is placed on the normal 1-alkenes as feedstocks, other lower olefins, such as 2-butene or isobutylene, are readily employed as starting materials due to rapid isomerization over the acidic zeolite catalysts. At conditions chosen to maximize heavy distillate and lubricant range products (C.sub.20.sup.+), the raw aliphatic product is essentially mono-olefinic. Overall branching is not extensive and may occur at spaced positions within the molecule.
The viscosity index of a hydrocarbon lube oil is related to its molecular configuration. Extensive branching in a molecule usually results in a low viscosity index. It is believed that two modes of oligomerization/polymerization of olefins can take place over acidic zeolites, such as HZSM-5. One reaction sequence takes place at Bronsted acid sites inside the channels or pores, producing essentially linear materials. The other reaction sequence occurs on the outer surface, producing more branched material. By decreasing the surface acid activity of such zeolites, fewer highly branched products with low VI are obtained.
Several techniques may be used to increase the relative ratio of intra-crystalline acid sites to surface active sites. This ratio increases with crystal size due to geometric relationship between volume and superficial surface area. Deposition of carbonaceous materials by coke formation can also shift the effective ratio, as disclosed in U.S. Pat. No. 4,547,613. However, enhanced effectiveness is observed where the surface acid sites of small crystal zeolites are reacted with a chemisorbed trialkyl pyridine, such as collidine.
It is a main object of this invention to provide an improved process for upgrading olefins to valuable lubricant quality product. Significantly improved linearity can be achieved by employing a catalyst comprising a medium pore shape selective siliceous zeolite with a surface that has been substantially inactivated with a sterically hindered nitrogenous base, such as a trialkyl pyridine compound.